Electron gun



May 10, 1960 G. R. BREWER ELECTRON cum 2 Sheets-Sheet '1 Filed July 18,1955 44440416 650M: 6 flan if,

I a W anuanf G. R. BREWER ELECTRON GUN May 10, 1960 2 Sheets-Sheet 2Filed 'July 18, 1955 :iaaamua 16140 Z a w 0 1 a I a a ELECTRON GUNGeorge R. Brewer, Palos Verdes Estates, Calif., assignor to HughesAircraft Company, Culver City, Calif, a corporation of DelawareApplication July 18, 1955, Serial No. 522,736 '3 Claims. (Cl. BIS-3.5)

,wave structures employed in tubes of this type, however,

it is generally necessary that an increase in the current flow beaccomplished by increasing the current density of the electron beam. Asthe current that can be emitted from a unit area of cathode emittingsurface soon reaches saturation, it is the present practice to employwhat is known as a converging beam electron gun. This type of electrongun incorporates a cathode having a large electron emitting surface. Inoperation, the electrons emitted from this large emitter surface areconverged into an electron beam of sufliciently small crossscctionalarea to be accommodated by the slow-wave structures.

The collimated magnetic field generally used for focusing andconstraining the electron beam in the travelingwave tube, may, however,tend to. limit the effectiveness of a converging beam electron gun. Thatis, a portion of the electrons emitted from the cathode have transversecomponents of emission velocity due to the effectof the magnetic linesof force'of the focusing field on the emitted electrons, particularly inthe regions about the outer periphery of cathode emitter surface. Thesetransverse velocity components of the electrons produce a variation incurrent density in the electron beam which is undesirable.

In accordance with the present invention, a converging beam electron gunis provided wherein a converging magnetic field is produced between thecathode and the comopening which is adapted to receive the elongatedportion of the tube envelope. A converging beam electron gunincorporating a cathode composed of a ferromagnetic material is employedso that leakage flux is attracted from this opening to the ferromagneticcathode to produce the converging magnetic field necessary foreliminating the transverse velocity components in the stream electrons.

.In another embodiment of the device of the present invention, theentire length of the tube is immersed in a tcs Patent Vice magneticfocusing field that would normally be collimated throughout the lengthof the tube. According to the present invention, however, aferromagnetic cathode together with apparatus for intercepting andconverging the focusing field inwards toward the anode aperture of theelectron gun is provided to produce a converging mag netic field betweenthe cathode and the anode of the electron gun. In still anotherembodiment of the invention, a converging magnetic field from aferromagnetic cathode to the anode aperture of a converging beamelectron gun is produced by magnetomotive force developed by a windingon the cathode which can also serve to heat the cathode to its propertemperature for electron emission. g

It is therefore an object of the invention to provide an improvedelectron gun for use in traveling-wave tubes.

Another object of the invention is to provide apparatus for minimizingthe transverse velocity components of electrons emitted from the cathodeof an electron gun.

Still another object of the invention is to provide ap-.

paratusfor producing a magnetic field through the electron emittingsurface of the cathode in a converging beamelectron gun that is normalto said electron emitting surface.

The novel features which are believed to be characteristic of theinvention, both as to its organization and.

method of operation, together with further objects and advantagesthereof, will be better understood from the following descriptionconsidered in connection with the accompanying drawings in which severalembodiments of the invention are illustrated by way of example. It is tobe expressly understood, however, that the drawings'are for the purposeof illustration and description only, andv additional embodiments of thedevice of the present invention.

Referring now to Fig. 1 there is illustrated a first embodiment of thedevice of the present invention comprising an evacuated envelope 10which has an elongated portion 11 and an enlarged portion 12. Theenlarged portion 12 of envelope 10 disposed at the left extremity, asviewed in the drawing, houses an electron gun 14 for producing anelectron beam. A solenoid 16 directsv the electron beam along apredetermined path through the elongated portion 11 and a collectorelectrode 17 is disposed at the extremity of the path farthest from theelectron gun 14 to intercept and collect the electron beam. A helix 18is disposed about the predetermined electron path to propagate anelectromagnetic wave therealong,

and input and output waveguide sections 20, 22 are coupled to the helix18. V

several thousand volts negative with respect to ground by means ofconnections therefrom to the negative ter- 1 minal of a battery 34, thepositive terminal ofwhich' is I connected to ground. The acceleratinganode 30, on": the other hand, is maintained at a potential ofthe-gorder; of 200 volts positive with respect to. ground. by means;

of a connection therefrom to the positive terminal of a V PatentedMay"), 1 9601:

battery 36, the negative terminal of which is referenced to ground.Inaddition to the above, the collector electrode 17 and the helix 18 aremaintained at potentials of the order of 200 volts positive and ground,respectively, by means of suitable connections to abattery 38 andground: The solenoid 16 is energized'witha directcurrent to'produce. amagnetic field of' from 600 to 1000 gauss' along the longitudinal axisof the tube 'by means of a connection across a battery 40.

In accordance with the invention, the cathode 24 comprises a member 42composed of a ferromagnetic material and having a circular concavesurface 44 of a sub stantially larger area than the cross-sectional areaof the electron beam. The member 42 is disposed in such a manner thatthe surface 144; faces helix 18 and is concentric with respect to thelongitudinal axis of the tube. The edges of the member 42 adjacent theconcave surface 44 are beveled .so that the angle between a radialtangent to the outer periphery of the surface 44 and the beveled surfaceis of the order of 90. In addition, the member 42 is made sufficientlythick so as to substantially decrease the reluctance of any magneticpath along the longitudinal axis of the tube. A layer 46 of emittermaterial is disposed over the concave surface 44 to provide a source ofelectrons. The layer 46 may, for example, be composed of sintered nickelpowder and alkaline earth carbonates, or other equivalent material. Inthat it is necessary to heat the layer 46 to temperatures of the orderof 800 centigrade to produce electron emission, it is necessary that theCurie temperature of the ferromagnetic material out of which member 42is composed be sufficiently high so that it retains its magneticproperties at an appropriate operating temperature. A Curie temperatureof the order of 900 centigrade would be suitable for a cathode operatingat 800 centigrade. A material out of which member 42 may be composed is,for example, cobalt as it has a sufficiently high Curie temperature anda vapor pressure that is less than that of nickel at the sametemperature. Nickel is a metal commonly used for making cathodes.

Operating in conjunction with the cathode 24 to produce a convergingmagnetic field commencing from the layer 46 of emitter material, is acylindrical magnetic shield 50 which encloses the solenoid 16 and theelongated portion 11 of the envelope The end portion of shield 50nearest the electron gun 14 has an aperture 52 disposed in the centerthereof of sufficient diameter to accommodate the elongated portion 11of the envelope 10. During operation, a magnetic field is produced alongthe longitudinal axis of the tube coextensive with the elongated portion11 of the envelope 10 to focus and constrain the electron beam along itspath in this region. The path of the magnetic flux constituting thisfield has a toroidal shape. That is, the field extends lengthwise alongthe tube within the solenoid 16 between the end portions of the shield50. Upon reaching the end portions, it proceeds in a radial direction tothe outer wall of the shield 50 whence it follows the low reluctancepath offered by this wall to form a closed path about the solenoid 16.

There is, however, some leakage flux which penetrates through theaperture 52 in the end portion of the shield 50 nearest the electron gun14. Due to the low reluctance presented by the. ferromagnetic member 42of cathode 24, this leakage flux threads the cathode 24 prior to closingupon its path aboutthe solenoid in a manner shown by dashed. lines. 54,55. In that the bevel about the outer periphery of concave; surface 44does not present a shorter path to the leakage flux, a maximum. of: theleakage flux threads the cathode 24. Thus the leakage flux constitutes aconverging magnetic field commencing from the. emitter surface of thecathode which considerably decreases the transverse velocity componentsof electrons emitted from the cathode 24.

The effect of producing a converging magnetic field commencing from theemitter surface is to bring electrons emitted from each elemental areaof the emitter surface of cathode 24 uniformly together to form anelectron beam that has a current distribution that approaches an idealbeam current distribution as represented by line 60 of Fig. 2. Theeffect of transverse emission velocities on current distribution isrepresented by line 62 of Fig. 2. It is generally desirable to avoidlarge-variations in current density across the electron beam so as toimprove the focusing characteristics or collimation of the electronstream; In the device of the present invention, it is apparent that thetrajectory of an electron is' influenced stronglyby'forces acting on itwhile it is near the cathode surface and thus moving relatively slowly.For this reason, if the transverse velocities of emission are to bereduced, it is preferable that the magnetic flux at the. cathodesurfacebe normal to this surface. is composed of a material which exhibitsferromagnetic properties at the operating temperature of the cathode, amagnetic field may be produced which is normal to the emitter surface ofthe cathode 24 irrespective of its size.

In an alternative embodiment. of the present invention, the electron gunportion of which is shown in Fig. 3, no magnetic shield 50 is employedand a focusing solenoid 16a is. made coextensive with the completelength of the tube including the enlarged portion thereof. The cathode24, focusing electrode 28 and'accelerating anode 30 of the electron gun14 are the same as those employed in the device of Fig. 1. However, anadditional horn 70 of ferromagnetic material has a throat portion whichis inserted in the aperture ofaccelerating anode 30 and expands radiallyoutwards from the electron beam along the direction of electron flow toa diameter substantially equal to or greater than the diameter of thecathode 24. The ferromagnetic horn 70 presents a low reluctance path tothe magnetic flux of the focusing field which causes a substantialportion of the flux to thread the horn 70. In this manner, a portion ofthe magnetic flux of the focusing field is concentrated about theaperture of the accelerating anode.

The cathode 24, as before, presents a low reluctance path to the flux ofa magnetic field. Thus the magnetic.

flux leaving the aperture of the accelerating anode is attracted towardsthe nearest surface of cathode 24. A typical flux distribution into theferromagnetic horn 70, from the born 70 to the cathode 24, and leavingthe cathode 24 is represented by lines 72, 73, and 74, respectively.Thus a converging magnetic field commencing from the emitter surface 96of the cathode 24 is produced. necessary to focus the electrons radiallyinwards electrostatically by means of a suitable'potential applied tothe focusing electrode 28 so as to minimize the number of electrons thatare intercepted by the horn 70 and accelerating electrode 30. Otherwisethe converging magnetic field represented by the lines 73 directthe'beam electrons towards the throat end of the horn'70.

In still another embodiment of the present invention illustrated in Fig.4, the cathode 24, focusing electrode 28, accelerating anode 30, andferromagnetic horn 70 similar to the device of Fig. 3 are employed. Inaddition, the cathode 24 is fitted with. an armature about which isdisposed a solenoid 82, whichservesto heat the-cathode 24 to its properemission temperature and to generate a magnetomotive force. Accordingly,the solenoid is energized with a directcurrent by means. of a connectionacross a battery 84. A low reluctance magnetic pathisprovided from theextremity of the armature 80 farthest from the emitter surface ofcathode 24 to the outer periph cry of the ferromagnetic. horn 70 by' acylinder 86 com- By the use of the member 42 which.

It is to be noted however, that in this case it is' the end portion ofthe cylinder 86 and the armature 80 as the cathode 24 must be maintainedat a potential that is different from that of accelerating anode 30. Thegeneration of a converging magnetic field commencing from the emittersurface of the cathode 24 to the throat of the horn 70 is similar tothat of the device of Fig. 3. Should this device be used in conjunctionwith a magnetic focusing field, however, the polarity of the currentenergizing the solenoid 82 should be such that the magnetic fieldbetween the cathode 24 and horn 70 aids that of the focusing field.

What is claimed is:

1. An electron gun capable of producing a high density electron beam ofpredetermnied cross-sectional area,

' said electron gun comprising a cathode including a support membercomposed of a ferromagnetic material for providing a concave supportsurface having beveled edges adjacent to said support surface, saidbeleved edges being approximately at an angle of 90 to said supportsurface at its junction therewith, a layer of emitter material disposedon said support surface having beveled edges, means for heating saidmember to a temperature less than the Curie temperature of saidferromagnetic material and sufficiently high to provide a source ofelectrons, and means for producing a magnetic field through saidcathode, said magnetic field being substantially perpendicular to saidsupport surface at each elemental area thereon.

2. An electron gun capable of producing a high density electron beam ofpredetermined cross-sectional area, said electron gun comprising acathode including a support member comprised of a ferromagnetic materialfor providing a support surface, said support surface being concavealong the axis of the electron beam and having a beveled edge around theconcave area, said beveled edge being at an angle approximately 90 withrespect to said support surface at its junction therewith, a layer ofemitter material disposed on the concave area of said support surface,means of heating said ferromagnetic material to a temperature less thanthe Curie temperature of said ferromagnetic material and sufficientlyhigh to provide a source of electrons, and means for producing amagnetic field having a central axis along the path of the electronbeam, said magnetic field extending through said support member andconverging from lines substantially perpendicular to the concave area ofsaid support surface into substantially parallel lines having thedesired cross-sectional area for the electron beam.

3. In a traveling-wave tube, apparatus for producing an electron beam ofpredetermined cross-sectional area comprising: a ferromagnetic cylinderencompassing a slow-wave structure of the traveling-Wave tube andincluding an aperture at an electron gun end thereof; a solenoiddisposed concentrically about and coextensive with the length of theelectron beam path to provide a constraining field along the electronbeam path, some of the field from the solenoid emerging in a leakageflux extending outside said ferromagnetic cylinder at the electron gunend thereof; a cathode structure spaced apart from said ferromagneticcylinder at the electron gun end thereof, said cathode including amember composed of ferromagnetic material providing an emitter supportsurface having a concave central surface along the axis of said beam andbeveled edges surrounding the periphery of said concave surface, saidbeveled edges being at an angle of approximately with respect to saidconcave surface at its junction therewith said ferromagnetic materialconverging the leakage flux extending outside said ferromagneticcylinder into the constraining field within said cylinder; and a layerof emitter material disposed on said support surface for emittingelectrons to be converged by said leakage flux into the electron beam ofpredetermined cross-sectional area prior to entering the ferromagneticcylinder through the aperture therein.

